Ann of Gorgeous Things just wrote a great post on how the right bra makes all the difference. Worth a read.
Funny she should write this now, because I just went for my first professional bra fitting in early January. And it has made SUCH a difference. I got the instant gratification factor of 10yrs/10lbs in 30 seconds by being in the right size bra. Virtually every piece of clothing--sewn and store bought--I own fits better now that everything is where it should be. (If you need proof, take a look at the Dotty Shirt I posted last week where I am wearing one of my new bras and compare it to any of my past dance shirts.)
But over the last few weeks, I have noticed how COMFORTABLE my new bras are. No more straps digging into my shoulders or falling off my shoulders. It did take a few days to get used to the way the new fit feels, but wow it feels wonderful now. It makes a difference in my posture too, now that the weight of my chest is carried by my rib cage, not my shoulders. I sit up straighter and don't hunch by the end of the day.
I also bought an athletic bra in the proper size, which has actually turned me into someone who enjoys jogging now that I don't eat my chest with every other stride. Someone should check those bras for crack, because I never thought I would voluntarily run anywhere unless being chased by scary people or in a drug induced state of mental confusion. :-)
For any in the Northern VA area, the name of the place is The Full Cup. They specialize in larger cup sizes, but have a full range of sizes. I can't recommend them enough.
And just to demonstrate how off a person can be--I thought I was a 38 D/DD. Turns out I am a 32G. Yeah. Whoops.